Our new paper “Proton transfer induced excited-state aromaticity gain for chromophores with maximal Stokes shifts” just appeard in Chemical Science.
Within this paper we ask the question of how to develop systems with pronounced excited-state aromaticity gain without having a strongly antiaromatic ground state. As a solution, we suggest excited-state intramolecular proton transfer (ESIPT).
It is by now fairly well understood how chromophore properties are affected by push/pull substituents and their degree of planarity. But how can we rationalise variations in the properties of planar chromophores that do not possess charge transfer character?
We were interested in understanding the apparent differences between these two isomeric molecules (called the Pechmann dyes).
Why is the T1 of PM5 (shown to the left) so much lower than the one of PM6 (shown to the right) making PM5 a powerful candidate for a singlet fission material whereas the S1/T1 gap of PM6 is too small for this purpose?
The answer is discussed in the new paper “Singlet Fission in Pechmann Dyes: Planar Chromophore Design and Understanding” that just appeared in JACS.
The overall lower excitation energies of PM5 vs PM6 can be understood by the fact that the S1 and T1 of the former are stabilised via excited-state aromaticity whereas the S0 of the latter profits from ground-state aromaticity.
The reason for the lower S1/T1 gap in PM5 is more subtle but also more fascinating pointing to a whole new way of viewing excitation energies. We were able to highlight the effect of the double bond conformation in influencing the exchange integral between the excited electron and hole, which ultimately leads to the variations in S1/T1 gap.
A recent JPCA article by Karadakov and Al-Yassiri highlights the differences in singlet and triplet aromaticity in naphthalene. To me this paper contains several striking observations:
We have discussed the excited states of naphthalene from an entirely different viewpoint in a recent J. Chem. Theory Comput. article. It would be fascinating to combine the two viewpoints.
Our new paper “Squarephaneic Tetraanhydride: A Conjugated Square-Shaped Cyclophane for the Synthesis of Porous Organic Materials” was just published in Angewandte Chemie. This paper, led by Florian Glöcklhofer from Imperial college, explores the redox chemistry, porosity, and chemical derivatives of a newly developed tetraanhyride based on a macrocycle with a formally antiaromatic ground state.
On the 14th of September Felix will give a talk at MAGIC 2022 (4th workshop on MAGnetically Induced Currents in molecules).
Title: “Bridging between the magnetic and molecular orbital pictures of excited-state aromaticity”
You can download the slides here.
A recent study, lead by Florian Glöcklhofer from Imperial College London, explores the effect of methoxy and thiomethyl subtitutions on a formally antiaromatic macrocycle. The corresponding paper “[2.2.2.2]Paracyclophanetetraenes (PCTs): cyclic structural analogues of poly(p‑phenylene vinylene)s (PPVs)” is available via Open Research Europe, 1, 111, 2012.
The above figure compares the orbitals and aromaticity descriptors for different charge and spin states. Importantly, the symmetry is broken in the T1 state, inhibiting Baird aromaticity. By comparison, the symmetry is retained for the neutral singlet, dianion, and dication states all of which exhibit aromaticity.
A recent study led by F. Glöcklhofer from Imperial College, London, investigates the properties of substituted conjugated macrocycles. The first (pre-review) version just appeared on Open Research Europe:
[2.2.2.2]Paracyclophanetetraenes (PCTs): cyclic structural analogues of poly(p‑phenylene vinylene)s (PPVs) by M. Pletzer, F. Plasser, M. Rimmele, M. Heeney, and F. Glöcklhofer.
Computations reveal the local and global aromaticity of the macrocycles in various electronic states by using the VIST method along with a discussion of the nodal structures of the orbitals.
Dylan has finished his MChem project entitled “Visualisation of Aromaticity and Antiaromaticity via the Computation of the Chemical Shielding on Multi-Dimensional Grids.” You can find his report here. The purpose of his project was to develop a convenient method for computing shielding tensors on a grid around a molecule. The developed code is available via github.
Below, an analysis of biphenylene is shown in the singlet (a) and triplet (b) state. For the singlet this representation highlights the aromaticity (red) of the benzene rings whereas the central 4-membered ring is found to be antiaromatic (blue). In the triplet (b), the whole molecule is found to be aromatic (red) according to Baird’s rule.
An analysis of norcorrole using either its doubly protonated form (a) or a nickel complex (b) highlights the antiaromaticity at the centre of this molecule whereas an aromatic pathway is found at the perimeter (see also [P. B. Karadakov, Org. Lett. 2020, 22, 8676]).
For more examples of how this type of analysis is used in the literature, see e.g., [Angew. Chemie – Int. Ed. 2020, 59, 19275] and [ChemPhysChem 2021, 22, 741].
How do macrocycles with [4n] electrons behave? Are there signatures of their formal antiaromaticity and how can their properties be tuned for practical applications? A recent study, led by Florian Glöcklhofer (Imperial College, London) endeavours to tackle these questions. A set of macrocycles based on [2.2.2.2]cyclophanetetraenes was synthesised, their redox and optical properties were measured, and a detailed computational analysis was performed.
Clear signatures of the unique properties of these macrocycles was found considering their large Stokes shifts (>1.5 eV) along with the ease of producing doubly charged states. A detailed computational analysis traces these properties back to the aromaticity of the excited and doubly charged states, respectively. In addition, it is illustrated how the properties of the macrocycles can be systematically varied with introduction of functional groups and variation of the aromatic units.
The study just appeared as a preprint on ChemRxiv: Functional Group Introduction and Aromatic Unit Variation in a Set of π-Conjugated Macrocycles: Revealing the Central Role of Local and Global Aromaticity.
Below, electron density difference plots for the charged states of the parent molecule paracyclophanetetraene are shown highlighting the cyclic symmetry of the electron attachment. The 2+/2- and 6+/6- states are aromatic whereas the 4+/4- singlet states are antiaromatic.
Version 2.4 of the TheoDORE wavefunction analysis package is available. Download the current version below.
Download the newest release of the TheoDORE wavefunction analysis program – TheoDORE 3.2 (22 July 2024)
